From: vladimir.murzin@arm•com (Vladimir Murzin)
To: linux-arm-kernel@lists•infradead.org
Subject: [RFC v2 PATCH 0/3] Fix dma_alloc_coherent() and friends for NOMMU
Date: Tue, 13 Dec 2016 15:02:30 +0000 [thread overview]
Message-ID: <58500D86.4070600@arm.com> (raw)
In-Reply-To: <ed07f3ca-8ce7-fb1b-2292-238ecf6460da@arm.com>
On 13/12/16 14:25, Robin Murphy wrote:
> On 13/12/16 14:14, Vladimir Murzin wrote:
>> On 13/12/16 14:07, Russell King - ARM Linux wrote:
>>> On Tue, Dec 13, 2016 at 01:45:01PM +0000, Vladimir Murzin wrote:
>>>> This patch set is trying to address the issue by providing region of
>>>> memory suitable for consistent DMA operations. It is supposed that such
>>>> region is marked by MPU as non-cacheable. Since we have MPU support in
>>>> Linux for R-class only and M-class setting MPU in bootloader, proposed
>>>> interface to advertise such memory is via "memdma=size at start" command
>>>> line option, to avoid clashing with normal memory (which usually comes
>>>> from dts) it'd be safer to use it together with "mem=" command line
>>>> option. Meanwhile, I'm open to suggestions for the better way telling
>>>> Linux of such memory.
>>>
>>> For those nommu systems where the MPU is not used, how do they allocate
>>> DMA memory without setting aside a chunk of memory?
>>>
>>> >From what I understand of the current nommu code, it would just use
>>> the normal page allocator for DMA memory allocation, so now requiring
>>> everything to fit the "nommu has mpu" case seems like it's going to
>>> break older nommu.
>>>
>>
>> Probably, it'd be better if we just fallback to dma-noop operations if there
>> is no dma region, i.e. assume that platform is coherent. We still need a way
>> to tell user that absence of such region can be reason of broken DMA.
>
> As I mentioned internally, I think it would be worth trying to use CMA
> for this, because dma_map_ops are already wired to try that first, and
> from what I can see it seems already set up to do precisely this via a
> "shared-dma-pool" reserved memory region (see rmem_cma_setup() in
> drivers/base/dma-contiguous.c) - mandating that for cached v7-M systems
> whilst letting cache-less/non-MPU systems automatically fall back to the
> normal page allocator in its absence would seem to solve all 3 cases.
Unfortunately,
config DMA_CMA
bool "DMA Contiguous Memory Allocator"
depends on HAVE_DMA_CONTIGUOUS && CMA
help
...
config CMA
bool "Contiguous Memory Allocator"
depends on HAVE_MEMBLOCK && MMU
select MIGRATION
and it blows up if I remove dependecy on MMU :(
Another option would be drivers/base/dma-coherent.c, but, IIUC, in this case
memory is reserved per device exclusively, so I'm in doubt if tiny M-class can
afford that...
>
> Other than the allocator issue, though, the rest of the refactoring does
> look nice.
Thanks for going through it!
Cheers
Vladimir
>
> Robin.
>
>>
>> Cheers
>> Vladimir
>>
>
>
next prev parent reply other threads:[~2016-12-13 15:02 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-13 13:45 [RFC v2 PATCH 0/3] Fix dma_alloc_coherent() and friends for NOMMU Vladimir Murzin
2016-12-13 13:45 ` [RFC v2 PATCH 1/3] ARM: NOMMU: introduce dma operations for noMMU Vladimir Murzin
2017-01-02 15:26 ` Benjamin Gaignard
2017-01-04 10:33 ` Vladimir Murzin
2017-01-06 13:58 ` Benjamin Gaignard
2017-01-09 13:54 ` Vladimir Murzin
2016-12-13 13:45 ` [RFC v2 PATCH 2/3] ARM: NOMMU: set ARM_DMA_MEM_BUFFERABLE for M-class cpus Vladimir Murzin
2016-12-13 13:45 ` [RFC v2 PATCH 3/3] ARM: dma-mapping: remove traces of NOMMU code Vladimir Murzin
2016-12-13 14:07 ` [RFC v2 PATCH 0/3] Fix dma_alloc_coherent() and friends for NOMMU Russell King - ARM Linux
2016-12-13 14:14 ` Vladimir Murzin
2016-12-13 14:25 ` Robin Murphy
2016-12-13 15:02 ` Vladimir Murzin [this message]
2016-12-13 18:32 ` Robin Murphy
2016-12-14 10:15 ` Vladimir Murzin
2016-12-13 14:33 ` Szemző András
2016-12-13 15:04 ` Vladimir Murzin
2016-12-16 14:57 ` Alexandre Torgue
2016-12-16 15:00 ` Vladimir Murzin
2016-12-16 15:33 ` Alexandre Torgue
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